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MInISPlITS ARe gAInIng PoPUlARITy, And FoR good ReASon For example, the Mitsubishi Mr. Slim Hyper-Heat unit The mosT promising heaTing sysTem for small houses is the ductless minisplit heat pump. A ductless minisplit is a type of air-source heat pump. Unlike a PTHP, which is all in one package, a ductless minisplit uses two major components. A condenser unit, which sits outside the house, is connected by copper tubing that circulates refrigerant to one or more indoor, wall-mounted blower units. With the proper valves and controls, ductless minisplits can supply space heat as well as cooling. In recent years, some manufacturers have improved and altered the compressors on these units to improve their heating efficiency significantly. Some ductless minisplits can now be used for heating in very cold climates. According to energy consultant Marc Rosenbaum, “Minisplits are cheap. They are a packaged system, so they don’t require much engineering design, are easy to install, and are a good match for low-load houses with renewable power.” The nominal heat- output rating listed in the specs of a ductless minisplit is calculated at an outdoor temperature of 47°F. The unit’s heat output drops with the outdoor temperature, so it’s important to check low-temperature performance when choosing a unit. heat pumps—that rarely make sense for small homes. Although in-floor radiant systems are a good way to heat a poorly insulated house, they are overkill and a waste of money in a small, tight house. If your goal is simplicity, there’s no reason to invest $12,000 or more on a boiler, one or more circulators, and hundreds of feet of tubing just to supply 15,000 Btu/hour on the coldest day of the year. If you need only a small amount of space heat, it’s equally unwise to invest in a ground-source heat-pump system, which usually costs at least $18,000. 58 FINE HOMEBUILDING (model PUZ-HA36nHA) has a nominal heat-output rating of 38,000 Btu/hour. According to the manufacturer, at an outdoor temperature of –13°F, its heat output drops 21%, to 30,000 Btu/hour. If your heating-design temperature is –13°F, you would assign the unit an output rating of 30,000 Btu/hour, not 38,000 Btu/hour. Another ductless minisplit, the Quaternity unit from daikin (model FTXg15HVJU), has a heat-output rating of 17,890 Btu/hour at 43°F. At –4°F, however, its heat output drops to 7310 Btu/hour. As long as the heating-system designer sizes the unit so that it will meet the building’s heat load at the design temperature, there’s no reason it won’t keep a home comfortable—even when the temperature drops well below zero. If you’re worried that a minisplit heat pump won’t be adequate when the thermometer bottoms out in January, remember: • The coldest temperature of the year is reached for only a few hours a year. • Tight, superinsulated homes lose heat very slowly, even during power outages. Unless a cold snap lasts for many days, most superinsulated homes won’t lose much heat. • in very cold temperatures, turning off the ventilation system will help a building to stay warm. • if you’re really worried about prolonged subzero cold snaps, one or two electric-resistance baseboard units provide cheap insurance. If you like the idea of a minisplit heat pump but want a ducted system to avoid having fan-coil units in your walls, check out the ducted minisplit units offered by some manufacturers (including daikin). The money required for in-floor radiant piping or a ground-source heat pump would be better invested in improvements to the building envelope—for example, improved air-sealing, more insulation, or high-quality triple-glazed windows. Builders achieving the Passive House standard have demonstrated the many advantages of superinsulation; if your building envelope falls short of Passive House performance levels—and it probably does—then envelope improvements usually make more sense than an investment in expensive heating equipment. No matter what type of heating equip- ment you choose, the first step should always be a thorough, accurate calculation of your home’s design heat load. Even contractors who do perform a Manual J calculation—a method published by the Air Conditioning Contractors of America (ACCA)—rarely bother to input all the necessary information without fudging and adding unnecessary “safety factors.” To avoid the typical result— oversize heating equipment—an accurate heat-loss calculation is essential. □ Martin Holladay is a contributing editor. Top drawing facing page: Martha Garstang Hill. Photos facing page: top, courtesy of Carter Scott; bottom, Justin Fink.